Copyright 2010 by Pearson Education Building Java Programs Chapter 8 Lecture 8-3: Constructors; Encapsulation reading: self-checks: #13-18, exercises: #5, 9, 14

Copyright 2010 by Pearson Education 2 The toString method reading: 8.6 self-check: #18, exercises: #9, 14

Copyright 2010 by Pearson Education 3 Printing objects By default, Java doesn't know how to print objects: Point p = new Point(); p.x = 10; p.y = 7; System.out.println("p is " + p); // p is // better, but cumbersome; p is (10, 7) System.out.println("p is (" + p.x + ", " + p.y + ")"); // desired behavior System.out.println("p is " + p); // p is (10, 7)

Copyright 2010 by Pearson Education 4 The toString method tells Java how to convert an object into a String Point p1 = new Point(7, 2); System.out.println("p1: " + p1); // the above code is really calling the following: System.out.println("p1: " + p1.toString()); Every class has a toString, even if it isn't in your code. Default: class's object's memory address (base 16)

Copyright 2010 by Pearson Education 5 toString syntax public String toString() { code that returns a String representing this object ; } Method name, return, and parameters must match exactly. Example: // Returns a String representing this Point. public String toString() { return "(" + x + ", " + y + ")"; }

Copyright 2010 by Pearson Education 6 Object initialization: constructors reading: 8.4 self-check: #10-12 exercises: #9, 11, 14, 16

Copyright 2010 by Pearson Education 7 Initializing objects Currently it takes 3 lines to create a Point and initialize it: Point p = new Point(); p.x = 3; p.y = 8; // tedious We'd rather specify the fields' initial values at the start: Point p = new Point(3, 8); // better! We are able to this with most types of objects in Java.

Copyright 2010 by Pearson Education 8 Constructors constructor: Initializes the state of new objects. public type ( parameters ) { statements ; } runs when the client uses the new keyword no return type is specified; it implicitly "returns" the new object being created If a class has no constructor, Java gives it a default constructor with no parameters that sets all fields to 0.

Copyright 2010 by Pearson Education 9 Constructor example public class Point { int x; int y; // Constructs a Point at the given x/y location. public Point(int initialX, int initialY) { x = initialX; y = initialY; } public void translate(int dx, int dy) { x = x + dx; y = y + dy; }... }

Copyright 2010 by Pearson Education 10 Tracing a constructor call What happens when the following call is made? Point p1 = new Point(7, 2); public Point(int initialX, int initialY) { x = initialX; y = initialY; } public void translate(int dx, int dy) { x += dx; y += dy; } x y p1

Copyright 2010 by Pearson Education 11 Client code, version 3 public class PointMain3 { public static void main(String[] args) { // create two Point objects Point p1 = new Point(5, 2); Point p2 = new Point(4, 3); // print each point System.out.println("p1: (" + p1.x + ", " + p1.y + ")"); System.out.println("p2: (" + p2.x + ", " + p2.y + ")"); // move p2 and then print it again p2.translate(2, 4); System.out.println("p2: (" + p2.x + ", " + p2.y + ")"); } OUTPUT: p1: (5, 2) p2: (4, 3) p2: (6, 7)

Copyright 2010 by Pearson Education 12 Multiple constructors A class can have multiple constructors. Each one must accept a unique set of parameters. Exercise: Write a Point constructor with no parameters that initializes the point to (0, 0). // Constructs a new point at (0, 0). public Point() { x = 0; y = 0; }

Copyright 2010 by Pearson Education 13 Common constructor bugs 1. Re-declaring fields as local variables ("shadowing"): public Point(int initialX, int initialY) { int x = initialX; int y = initialY; } This declares local variables with the same name as the fields, rather than storing values into the fields. The fields remain Accidentally giving the constructor a return type: public void Point(int initialX, int initialY) { x = initialX; y = initialY; } This is actually not a constructor, but a method named Point

Copyright 2010 by Pearson Education 14 Encapsulation reading: self-check: #13-17 exercises: #5

Copyright 2010 by Pearson Education 15 Encapsulation encapsulation: Hiding implementation details from clients. Encapsulation forces abstraction. separates external view (behavior) from internal view (state) protects the integrity of an object's data

Copyright 2010 by Pearson Education 16 Private fields A field that cannot be accessed from outside the class private type name ; Examples: private int id; private String name; Client code won't compile if it accesses private fields: PointMain.java:11: x has private access in Point System.out.println(p1.x); ^

Copyright 2010 by Pearson Education 17 Accessing private state // A "read-only" access to the x field ("accessor") public int getX() { return x; } // Allows clients to change the x field ("mutator") public void setX(int newX) { x = newX; } Client code will look more like this: System.out.println(p1.getX()); p1.setX(14);

Copyright 2010 by Pearson Education 18 Point class, version 4 // A Point object represents an (x, y) location. public class Point { private int x; private int y; public Point(int initialX, int initialY) { x = initialX; y = initialY; } public int getX() { return x; } public int getY() { return y; } public double distanceFromOrigin() { return Math.sqrt(x * x + y * y); } public void setLocation(int newX, int newY) { x = newX; y = newY; } public void translate(int dx, int dy) { setLocation(x + dx, y + dy); }

Copyright 2010 by Pearson Education 19 Benefits of encapsulation Abstraction between object and clients Protects object from unwanted access Example: Can't fraudulently increase an Account 's balance. Can change the class implementation later Example: Point could be rewritten in polar coordinates (r, θ) with the same methods. Can constrain objects' state (invariants) Example: Only allow Account s with non-negative balance. Example: Only allow Date s with a month from 1-12.

Copyright 2010 by Pearson Education 20 The keyword this reading: 8.7

Copyright 2010 by Pearson Education 21 The this keyword this : Refers to the implicit parameter inside your class. (a variable that stores the object on which a method is called) Refer to a field: this. field Call a method: this. method ( parameters ); One constructor this( parameters ); can call another:

Copyright 2010 by Pearson Education 22 Variable shadowing shadowing: 2 variables with same name in same scope. Normally illegal, except when one variable is a field. public class Point { private int x; private int y;... // this is legal public void setLocation(int x, int y) {... } In most of the class, x and y refer to the fields. In setLocation, x and y refer to the method's parameters.

Copyright 2010 by Pearson Education 23 Fixing shadowing public class Point { private int x; private int y;... public void setLocation(int x, int y) { this.x = x; this.y = y; } Inside setLocation, To refer to the data field x,say this.x To refer to the parameter x,say x

Copyright 2010 by Pearson Education 24 Calling another constructor public class Point { private int x; private int y; public Point() { this(0, 0); // calls (x, y) constructor } public Point(int x, int y) { this.x = x; this.y = y; }... } Avoids redundancy between constructors Only a constructor (not a method) can call another constructor